1. Field of the Invention
The present invention relates to control of charge storage time in a still video camera (electronic still camera).
2. Description of the Related Art
Still video cameras, which have recently become increasingly popular, typically utilize mechanical shutters as means for controlling the charge storage time of their image sensors. The charge storage time is the time period during which the light required to form an image is incident on a solid-state image sensor, and has conventionally been controlled by opening and closing the mechanical shutter.
Such a conventional arrangement may be regarded as an arrangement in which film for a silver-salt camera is merely replaced with an image sensor, and in such an arrangement, the aperture diameter is controlled by means of the diaphragm disposed in the lens.
However, the shutter blades of the mechanical shutter generally show variations in operation, and a complicated mechanism is needed to attain an operation of sufficient accuracy. The upper limit of the shutter speed of the mechanical shutter is generally on the order of 1/1000 seconds. In addition, even if an aperture-priority mode is set to control exposure with shutter time, it is impossible to control the exposure with high accuracy.
A further advanced arrangement is known in which, during high-speed continuous shooting, exposure time (charge storage time) is controlled by means of an electronic shutter with the mechanical shutter kept fully open.
However, in the image sensing apparatus for still image which uses, as described above, the image sensor the latitude of which is narrow compared to silver-salt film, it is not easy to gain sufficient accuracy in the control of both the aperture diameter and the exposure time, resulting in the disadvantage that a costly arrangement is required.
To cope with the problem of cost increase, one method is considered in which the accuracy of the aperture diameter is improved by stepwise changing the aperture diameter within a restricted number of steps without continuously changing the aperture diameter. This method has the disadvantages that it is necessary to incorporate a shutter and a diaphragm separately and that a high degree of shutter accuracy is still needed.
It may also be considered possible to overcome the aforesaid problem of cost increase by using a set of blades as a diaphragm and a shutter to reduce the number of parts used. However, to control accurately the opening size formed by the blades, it is necessary to lower the operating speed of the blades, thus leading to the disadvantage that a high-speed shutter operation is difficult to achieve, which is unfavorable for operational characteristics.
This disadvantage not only makes it difficult to achieve a high-speed shutter operation but also constitutes a cause of bringing about the disadvantage of reducing the range (of subject luminance) in which a flash unit can be utilized for so-called fill-in flash photography in which flashing is used to prevent a subject against the high-luminance background from becoming dark to an imperceptible extent, so that the probability that a situation in which such a backlighting compensation is impossible is encountered becomes high.
Another arrangement which utilizes a motor to drive the blades employs a method of stabilizing the opening size by stopping the blades by ceasing the supply of electricity to the blade-opening motor or by energizing it in the reverse direction. In such a method, the moving speeds of the blades vary depending on a voltage applied to the motor, and even if a blade-stopping operation is carried out for the same aperture diameter, an error occurring until the blades actually stop varies and the aperture diameter does not stabilize. To avoid this disadvantage, it is necessary to use means for calculating a correct exposure time by again measuring light which passes through an actual aperture after the stabilization of the aperture diameter, means for stabilizing a voltage applied to the motor to stabilize the aperture diameter, etc. Therefore, a simple system will not be obtained.